The present invention concerns liquid filtration apparatus of the type including an open tank adapted to receive a liquid to be filtered, with a media belt extending over a "vacuum box" having a perforate cover located in a section of the bottom of the tank. Liquid is drawn through the media in order to be filtered and into the vacuum box for recirculation to the system using the liquid. The media belt is periodically indexed to bring a fresh section of the media belt over the vacuum box and to carry accumulated solids out of the tank.
Various arrangements have heretofore been employed in such filters. A chain conveyor is often employed, having chain loops running along either edge of the belt, advance of the chain loops causing advance of the media belt by a frictional engagement of segments of the chain loops lying atop the media belt edges on the tank bottom. These chain loop segments also seal the edges of the media belt by their weight. Chain flights extending across the belt are used to increase frictional engagement and to enable large masses of accumulated solids atop the media belt to be more easily advanced out of the tank.
The media belt has sometimes consisted of a disposable material such as paper or woven material which is fed off a roll at one end of the tank into the space between the conveyor chain and passed into a collection receptacle. U.S. Pat. No. 3,087,620 describes such a filter.
It is sometimes desirable to use a recirculated permanent media belt, each segment of which must be cleansed prior to being repositioned over the vacuum box.
Heretofore, where a chain conveyor is used, such permanent media belts have been recirculated through the same path as the chain conveyor. A problem with this approach is that the media belt typically must be narrower than the driver sprocket spacing, and must be attached to the chain since a friction drive is precluded. Also, a second more closely spaced pair of chain loops must be used to insure proper sealing of the belt since the belt must be narrower than the conveyor to avoid having to pass over the drive sprockets. This complexity increases the cost of the media belt and greatly increases the time required to change a media belt when replacement becomes necessary.
Such a filter is described in U.S. Pat. No. 4,514,301 issued on Apr. 30, 1985 for a "Continuous Media Filter" in which the conveyor chain loops and belt are circulated beneath the tank.
In disposable media filters, the chain conveyor loops typically return by being passed back over the top of the tank. See U.S. Pat. Nos. 4,396,505 and 3,221,885 for examples of these disposable media filter apparatus designs.
Another disadvantage of connected media belt and chain conveyors is that cleaning of each is made more difficult as scraping the chain conveyor flights and belt surface is made difficult with these elements always remaining positioned against each other.
Another arrangement heretofore employed has utilized a permanent belt returning over the top of the tank. This arrangement results in the open top of the tank being covered, making observation of conditions and access to the tank interior difficult.
In such permanent media belt filters heretofore employed, it has been difficult to employ a disposable media belt in combination with a permanent media belt, as the disposable belt must be inserted between the chain conveyor and the permanent media, which is not easily accomplished in the prior designs.